MC9S08AC60 Datasheet, PDF (137/348 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 9 Analog-to-Digital Converter (S08ADC10V1)
9.2.3 Temperature Sensor
The ADC module includes a temperature sensor whose output is connected to one of the ADC analog
channel inputs. Equation 9-1 provides an approximate transfer function of the temperature sensor.
Temp = 25 - ((V_TEMP -V_TEMP25) ÷ m)
Eqn. 9-1
where:
— V_TEMP is the voltage of the temperature sensor channel at the ambient temperature.
— V_TEMP25 is the voltage of the temperature sensor channel at 25°C.
— m is the hot or cold voltage versus temperature slope in V/°C.
For temperature calculations, use the V_TEMP25 and m values from the ADC Electricals table.
In application code, the user reads the temperature sensor channel, calculates V_TEMP, and compares to
V_TEMP25 . If V_TEMP is greater than V_TEMP25 the cold slope value is applied in Equation 9-1. If V_TEMP is
less than V_TEMP25 the hot slope value is applied in Equation 9-1.
To improve accuracy , calibrate the bandgap voltage reference and temperature sensor.
Calibrating at 25 °C will improve accuracy to 4.5°C.
Calibration at 3 points, -40 °C, 25 °C, and 125 °C will improve accuracy to 2.5 °C. Once calibration has
been completed, the user will need to calculate the slope for both hot and cold. In application code, the
user would then calculate the temperature using Equation 9-1 as detailed above and then determine if the
temperature is above or below 25 °C. Once determined if the temperature is above or below 25 °C, the user
can recalculate the temperature using the hot or cold slope value obtained during calibration.
For more information on using the temperature sensor, consult AN3031.
MC9S08AC60 Series Data Sheet, Rev. 2
Freescale Semiconductor
137